I. Field of the Invention
This invention relates to cellular radio systems, and in particular to channel-assignment arrangements for such systems.
II. Related Art and Other Considerations
In cellular radio systems a number of base stations are provided which, between them, provide radio coverage for the area to be served by the system. A mobile radio unit within the service area can make radio contact with the base station having the strongest signal, which is usually the nearest. As the mobile unit travels through the service area, the mobile unit may get further from this base station so that the signal strength on this first radio link deteriorates. Cellular systems provide arrangements whereby communication can then be established with a second base station, the link with the first being relinquished. This process is known as "handover". Each mobile unit requires a separate channel to be assigned in order to communicate with a base station. A channel is assigned to the link between the mobile unit and the base station when communication is to be set up, the channel being selected from chose allocated for use by the base station and available for setting up a link (i.e. not currently assigned to a link. The different channels may be frequencies, time slots within a frequency, or some other division of the spectrum.
In order to avoid co-channel interference, particularly at boundaries between cells, each cell must have available to it a channel or group of channels which is different from those allocated to its neighbours. Considerable research has gone into optimising re-use patterns of these channels, to determine how close together two cells using the same channels can be without causing significant co-channel interference, thereby allowing the most efficient use of the spectrum. However, a major problem is that different cells have different traffic levels at different times of day, not always on a predictable basis, and the capacity requirements have to be based on the busiest period of each cell, even when these peak times do not coincide. This can result in channels being idle in one cell despite there being heavy demand in nearby cells.
To take a particular example, a cell serving a transport centre such as a railway station is likely to be at its busiest during the peak travel hours. The number of channels which need to be allocated to that cell in order to achieve an acceptable call success rate is determined by the call traffic density at the peak time. None of these channels can be re-used in any other neighbouring cell (i.e. one not necessarily adjacent to the first cell, but close enough for co-channel interference to be a possibility). However, a neighbouring cell may have a peak traffic density at another time of day. This must also be given a channel allocation sufficiently great to handle its peak density. The result is an inefficient use of the available channels, as at any time of day there is spare capacity in one or other of the cells.
Dynamic reconfiguration of the basic channel allocation scheme in neighbouring cells, in order to match changes in demand, is liable to have knock-on effects on further cells unless strictly controlled. Any such reconfiguration must also take place promptly to react to sudden surges in demand. Regular shifts in demand could be handled automatically by switching channels from one cell to another at predetermined times of day but this can only cope with predictable shifts in demand. Moreover, if a block of channels is shifted at a set time, any calls operating on a channel when it is shifted could be lost.
It is known from PCT patent application number WO91/01073, in the name of Telecom Securicor Cellular Radio Ltd, to provide a cellular radio system using time division multiple access in which each sector of a cell structure is subdivided into two sub-sectors. When communication is set up between a mobile unit and the base station the required time slot is transmitted only within the sub-sector in which the mobile unit is located. This reduces the amount of power required and reduces the possibility of co-channel interference since a more highly directional antenna can be used. The receive antenna at the base station can also be made more directional with similar benefits.
This prior art arrangement reduces the likelihood of co-channel interference and reduces the transmitter power required at the base station. However, it does not increase the overall capacity of any sector.
Because it assigns individual time slots it is also limited to a situation where the base stations are co-located, since synchronisation would otherwise be a problem.
In areas in which for reasons of topography and/or radio traffic density the basic cell structure would not provide an adequate service, it has been proposed to provide microcells. A microcell covers a smaller area than that covered by a typical macrocell of the basic cell structure. Consequently microcell base stations can be of lower power than the base stations of macrocells. Nevertheless, there is a problem in selecting channels for use by the microcell which do not interfere with any channels in use in nearby cells.